Super hard alloy baseplate outer circumference cutting blade and manufacturing method thereof

ABSTRACT

The disclosed cemented carbide base outer blade cutting wheel comprises a base in the form of an annular thin disc of cemented carbide, and a blade section on the outer periphery of the base. The blade section contains: diamond and/or CBN abrasive grains pre-coated with a magnetic material; a metal or alloy bond formed by electroplating or electroless plating for bonding abrasive grains together and to the base; a resin infiltrated between abrasive grains and between abrasive grains and the base, said resin being a thermoplastic resin having a melting point of up to 350° C. or a thermoset resin obtained by curing a liquid thermosetting resin composition having a curing temperature of up to 350° C. The method for manufacturing said outer blade cutting wheel is also disclosed.

TECHNICAL FIELD

This invention relates to a cemented carbide base outer-diameter bladecutting wheel suited for cutting rare earth sintered magnets, and amethod for preparing the same.

BACKGROUND ART

For cutoff machining of rare earth sintered magnet blocks (or permanentmagnet blocks), various cutting techniques such as outer diametercutting, inner diameter cutting and wire saw cutting are implemented.Among others, outer-diameter blade cutting wheels are most widelyemployed. The outer-diameter blade cutting technique has many advantagesincluding less expensive cutting tools, a relatively low cuttingallowance associated with cemented carbide blades, a good dimensionalaccuracy of cut pieces, and a relatively high machining speed. Owing tothese advantages and improved mass productivity, the outer-diameterblade cutting technique is widely used in the cutting of rare earthsintered magnet blocks.

outer-diameter blade cutting wheels for cutting rare earth sinteredmagnets are disclosed in JP-A H09-174441, JP-A H10-175171, and JP-AH10-175172 as comprising a cemented carbide base having an outerperiphery to which diamond or CBN abrasive grains are bonded withphenolic resins, nickel plating or the like. Since the base made ofcemented carbide is improved in mechanical strength over prior art alloytool steel or high-speed steel, there are achieved an improvement in themachining accuracy, an improvement in the yield of pieces due to reducedallowance by the use of thin blades, and a reduction of machining costdue to high-speed machining.

While outer-diameter blade cutting wheels using cemented carbide basesare improved in cutting and working performances over the prior artouter-diameter blade cutting wheels, the market imposes a continuingdemand for cost reduction. It would be desirable to have ahigh-performance cutting wheel capable of machining at a high accuracyand high speed.

CITATION LIST Patent Document

-   Patent Document 1: JP-A H09-174441-   Patent Document 2: JP-A H10-175171-   Patent Document 3: JP-A H10-175172-   Patent Document 4: JP-A 2005-193358-   Patent Document 5: JP-A H07-207254-   Patent Document 6: JP 2942989-   Patent Document 7: JP-A 2005-219169-   Patent Document 8: WO 96/23630-   Patent Document 9: JP-A 2009-172751

SUMMARY OF INVENTION Technical Problem

The applicant previously proposed a technique of bonding diamondabrasive grains to the periphery of an annular cemented carbide basewith a resin such as phenolic resin and a technique of bonding diamondor CBN abrasive grains to the periphery of an annular cemented carbidebase with a metal bond having an appropriate Young's modulus (JP-A2009-172751).

The outer blade cutting wheel for use in the cutting of rare earthsintered magnet blocks is composed of two sections, a base and a bladesection. Now that the base accounting for the majority of the cuttingwheel is made of high-modulus cemented carbide, the cutting wheel isimproved in mechanical strength and hence, in cutting accuracy over theprior art cutting wheels having alloy tool steel and high-speed steelbases. The switch to a metal bond having an appropriate Young's modulus,combined with the cemented carbide base, improves the mechanicalstrength of the overall cutting wheel, thereby achieving threeperformance improvements, an improvement in the machining accuracy, animprovement in the yield of material due to the use of thin blades, anda reduction of machining cost due to high cutting speed, as comparedwith the prior art outer blade cutting wheels of the resin bond typeusing phenolic resins or polyimide resins as the bond for abrasivegrains.

Cemented carbide base outer blade cutting wheels can be manufactured byproducing a magnetic field near the outer periphery of the cementedcarbide base, the magnetic field acting on abrasive grains pre-coatedwith a magnetic material so as to magnetize the coating on abrasivegrains, thereby attracting the abrasive grains toward the base outerperiphery, and effecting plating in this state, thereby bonding theabrasive grains to the outer periphery. The method reduces the cost ofmanufacture of outer blade cutting wheels.

The cemented carbide base outer blade cutting wheel produced by theabove-mentioned method is an outer blade cutting wheel featuring highperformance. When a rare earth sintered magnet block is cut into magnetpieces by the wheel, sometimes the dimensional accuracy is aggravatedbecause the block can be obliquely cut, or cutting marks by the wheel beleft on the cut surface of a magnet piece. Specifically, when a cementedcarbide base outer blade cutting wheel having an outer diameter of80-200 mm, a bore diameter of 30-80 mm, and a thickness of 0.1-1.0 mm,for example, is used and operated to perform high-speed, high-loadcutting at a machining volume per unit time of at least 200 mm³/min, adimensional tolerance may exceed 50 μm. If the dimensional accuracy isaggravated, some remedies are necessary. For example, the magnet piecesmust be subjected to an additional step of precision grinding the cutsurface such as by lapping. The outer blade cutting wheel must bedressed using a grinding wheel or the cutting conditions be altered.

This becomes a barrier in the machining of magnet pieces which aresuited for use in motors in which a strict management of the clearancebetween a yoke and a magnet is required such as linear motors and harddisc voice coil motors (VCM) and which require both a high dimensionalaccuracy (including the flatness of cut surface) and a reduction ofmanufacture cost.

An object of the invention is to provide a cemented carbide base outerblade cutting wheel capable of cutting a rare earth sintered magnetblock into pieces having a high dimensional accuracy, and a method forpreparing the outer blade cutting wheel at a low cost.

Solution to Problem

It is presumed that a phenomenon that a rare earth sintered magnet blockis obliquely cut takes place because the outer blade cutting wheel has ablade shape which is not laterally symmetric, allowing cutting operationto proceed in a direction of easy cutting, and because the outer bladecutting wheel is warped when it is mounted on a machining tool. It isalso presumed that a phenomenon that cut marks are left on magnet piecestakes place because when the outer blade cutting wheel which is cuttingthe magnet block obliquely for the above reason changes its traveldirection abruptly on the way of cutting operation, the cut surfacewhich is newly cut does not smoothly merge with the cut surface whichhas been cut, forming a step.

An abrupt change of the travel direction of the outer blade cuttingwheel during cutting operation occurs, for example, when the blade ofthe outer blade cutting wheel is, in part, deformed or spalled for somereason; when the blade edge abruptly changes its shape; when the bladeis deformed by the feed speed of the cutting wheel which is higher thanthe grinding speed of the blade, the internal stress induced in theblade by that deformation becomes greater than the external forceapplied from the workpiece to the blade, and as a consequence, the forcecausing deformation of the blade is released; and when the travel of theouter blade cutting wheel is interrupted by loading or glazing of thecutting groove with sludge formed during cutting operation or foreignmatter of external origin. To eliminate any cut marks which can formunder such conditions, it is effective that the blade edge does notabruptly change its shape, and that when any force is applied to theblade so as to change its travel direction during cutting operation, theblade is deformed to such an extent as to smoothly merge the cutsurfaces before and after the change.

A void problem arises in the outer blade cutting wheel in which abrasivegrains are bonded to a base by electroplating or electroless plating toform a blade section. Since the abrasive grains have a certain grainsize, the bonded abrasive grains are contacted only in part betweengrains and between grains and the base, and voids therebetween are notcompletely buried by plating. As a result, voids are left in the bladesection even after plating. That is, the blade section contains voids incommunication with the surface.

As long as the load applied to the outer blade cutting wheel duringcutting operation is low, high accuracy cutting is possible, even in thepresence of such voids, because the blade section does not undergosubstantial deformation by the force applied during cutting. However,where cutting is carried out under such a high load as to cause thecemented carbide base to be deformed, the blade edge can be in partdeformed or shed. An effective method for preventing the blade edge fromdeformation or shedding is by enhancing the strength of the blade edge.Since the blade section should have a sufficient elasticity to allow theblade section to deform to enable smooth mergence of cut surfaces aswill be described later, the mere enhancement of blade strength to beresistant to deformation fails to address the problem.

Making further investigations on the construction of the blade sectionthat meets both high strength and elasticity and the necessarymechanical properties of the blade section, the inventors have foundthat an effective blade section is obtained by utilizing voids betweenabrasive grains and between abrasive grains and the base, specificallyby letting a resin infiltrate in the voids. The resin infiltrate isformed by melting a thermoplastic resin, infiltrating and solidifying,or by infiltrating a liquid thermosetting resin composition and curing.An outer blade cutting wheel comprising a cemented carbide base and sucha blade section is effective in improving the dimensional accuracy ofmagnet pieces cut thereby. The technique of melting a thermoplasticresin, infiltrating and solidifying or the technique of infiltrating aliquid thermosetting resin composition and curing is effective for themanufacture of an outer blade cutting wheel featuring a high cuttingaccuracy and low cost. The invention is predicated on these findings.

In one aspect, the invention provides an outer blade cutting wheelcomprising a base in the form of an annular thin disc of cementedcarbide having a Young's modulus of 450 to 700 GPa, having an outerdiameter of 80 to 200 mm defining an outer periphery, an inner diameterof 30 to 80 mm, and a thickness of 0.1 to 1.0 mm, and a blade section onthe outer periphery of the base,

the blade section comprising diamond and/or CBN abrasive grainspre-coated with a magnetic material, a metal or alloy bond formed byelectroplating or electroless plating for bonding abrasive grainstogether and to the base, and a resin infiltrated between abrasivegrains and between abrasive grains and the base, said resin being athermoplastic resin having a melting point of up to 350° C. or athermoset resin obtained by curing a liquid thermosetting resincomposition having a curing temperature of up to 350° C.

In a preferred embodiment, the resin is at least one member selectedfrom the group consisting of acrylic resins, epoxy resins, phenolicresins, polyamide resins, polyimide resins, and modified resins of theforegoing. Also preferably, the resin has a Poisson's ratio between 0.3and 0.48.

In a preferred embodiment, the base has a saturation magnetization of atleast 40 kA/m (0.05 T).

In a preferred embodiment, the abrasive grains have an average grainsize of 10 to 300 μm. Also preferably, the abrasive grains have a massmagnetic susceptibility χg of at least 0.2.

In another aspect, the invention provides a method for manufacturing anouter blade cutting wheel comprising the steps of:

providing a base in the form of an annular thin disc of cemented carbidehaving a Young's modulus of 450 to 700 GPa, having an outer diameter of80 to 200 mm defining an outer periphery, an inner diameter of 30 to 80mm, and a thickness of 0.1 to 1.0 mm,

providing diamond and/or CBN abrasive grains pre-coated with a magneticmaterial,

placing a permanent magnet near the outer periphery of the base so thatthe magnetic field produced by the permanent magnet may act tomagnetically attract and hold the coated abrasive grains close to theouter periphery of the base,

electroplating or electroless plating a metal or alloy on the base outerperiphery and the coated abrasive grains being magnetically attractedand held, for bonding the abrasive grains together and to the base tofixedly secure the abrasive grains to the base outer periphery to form ablade section, and

letting a thermoplastic resin having a melting point of up to 350° C.infiltrate into any voids between abrasive grains and between abrasivegrains and the base or letting a liquid thermosetting resin compositionhaving a curing temperature of up to 350° C. infiltrate into any voidsbetween abrasive grains and between abrasive grains and the base andcure thereat.

In a preferred embodiment, the resin is at least one member selectedfrom the group consisting of acrylic resins, epoxy resins, phenolicresins, polyamide resins, polyimide resins, and modified resins of theforegoing. Also preferably, the infiltrating resin has a Poisson's ratiobetween 0.3 and 0.48.

In a preferred embodiment, the base has a saturation magnetization of atleast 40 kA/m (0.05 T).

In a preferred embodiment, the abrasive grains have an average grainsize of 10 to 300 μm. Also preferably, the abrasive grains have a massmagnetic susceptibility χg of at least 0.2.

In a preferred embodiment, the permanent magnet produces a magneticfield of at least 8 kA/m within a space extending a distance of 10 mm orless from the base outer periphery.

Advantageous Effects of Invention

Using the cemented carbide base outer blade cutting wheel, a rare earthmagnet block is cut into magnet pieces. With only the cutting operation,the magnet pieces are finished to a high dimensional accuracy. Anyfinishing following the cutting operation may be omitted. Rare earthmagnet pieces having a high dimensional accuracy are obtained at a lowcost.

The method for manufacturing the outer blade cutting wheel is costeffective.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates an outer blade cutting wheel in oneembodiment of the invention, FIG. 1A being a plan view, FIG. 1B being across-sectional view taken along lines B-B in FIG. 1A, and FIG. 1C beingan enlarged view of circle C (blade section) in FIG. 1B.

FIG. 2 is a perspective exploded view of one exemplary jig used in themethod.

FIG. 3 is an enlarged cross-sectional view of the outer portions of theholders sandwiching the base in FIG. 2.

FIGS. 4A to 4D are cross-sectional views of different embodiments of theblade section formed on the base.

FIG. 5 is a photomicrograph of a blade section of an outer blade cuttingwheel in Example 1 on its side surface.

FIG. 6 is a diagram showing cutting accuracy versus the number of magnetpieces cut using the outer blade cutting wheels of Examples 1 to 4 andComparative Example 1.

FIG. 7 is a diagram showing stress versus deformation of the bladesections of the outer blade cutting wheels of Examples 1 to 4 andComparative Example 1.

DESCRIPTION OF EMBODIMENTS

Referring to FIG. 1, the outer blade cutting wheel in one embodiment ofthe invention is illustrated as comprising a base 10 in the form of anannular thin disc made of cemented carbide and a blade section 20disposed on the outer periphery of the base 10. The blade section 20comprises diamond and/or CBN abrasive grains bonded with a metal ormetal alloy bond by electroplating or electroless plating.

The base 10 is in the form of an annular thin disc (differently stated,a doughnut-shaped thin plate having a center bore 12) having an outerdiameter of 80 to 200 mm, preferably 100 to 180 mm, defining an outerperiphery, an inner diameter of 30 to 80 mm, preferably 40 to 70 mm,defining the bore 12, and a thickness of 0.1 to 1.0 mm, preferably 0.2to 0.8 mm.

It is noted that the disc has a center bore and an outer circumferenceas shown in FIG. 1. Thus, the terms “radial” and “axial” are usedrelative to the center of the disc, and so, the thickness is an axialdimension, and the length (or height) is a radial dimension. Likewisethe terms “inside” or “inward” and “outside” or “outward” are usedrelative to the center of the disc or the rotating shaft of the cuttingwheel.

The base has a thickness in the range of 0.1 to 1.0 mm and an outerdiameter in the range of not more than 200 mm because a base of suchdimensions can be manufactured at a high accuracy and ensures consistentcut-off machining of a workpiece, typically a rare earth sintered magnetblock at a high dimensional accuracy over a long term. A thickness ofless than 0.1 mm leads to a likelihood of noticeable warpage independentof outer diameter and makes difficult the manufacture of a base at ahigh accuracy. A thickness in excess of 1.0 mm indicates an increasedcutting allowance. The outer diameter is up to 200 mm in view of thesize that can be manufactured by the existing technology of producingand processing cemented carbide. The diameter of the bore is set in arange of 30 to 80 mm so as to fit on the shaft of the cutoff machiningtool.

Examples of the cemented carbide of which the base is made include thosein which powder carbides of metals in Groups IVB, VB, and VIB of thePeriodic Table such as WC, TiC, MoC, NbC, TaC and Cr₃C are cemented in abinder matrix of Fe, Co, Ni, Mo, Cu, Pb, Sn or a metal alloy thereof, bysintering. Among these, typical WC—Co, WC—Ti, C—Co, and WC—TiC—TaC—Cosystems are preferred. They should have a Young's modulus of 450 to 700GPa. Also, those cemented carbides which have an electric conductivitysusceptible to plating or which can be given such an electricconductivity with palladium catalysts or the like are preferred. Whencemented carbides are given an electric conductivity with palladiumcatalysts or the like, well-known agents such as metallizing agents usedin the metallization of ABS resins may be employed.

With respect to magnetic properties of the base, a greater saturationmagnetization is preferred for holding abrasive grains to the base bymagnetic attraction. Even in the case of a lower saturationmagnetization, however, magnetic material-coated abrasive grains can bemagnetically attracted toward the base by controlling the position of apermanent magnet and the strength of a magnetic field. For this reason,a base having a saturation magnetization of at least 40 kA/m (0.05 T) issatisfactory.

The saturation magnetization of a base is determined by cutting a sampleof 5 mm squares out of a base having a given thickness, and measuring amagnetization curve (4πI-H) of the sample at a temperature of 24-25° C.by means of a vibrating sample magnetometer (VSM). The upper limit ofmagnetization values in the first quadrant is assigned as the saturationmagnetization.

The outer periphery of the base may advantageously be chamfered (beveledor rounded) in order to enhance the bond strength between the base andthe blade section which is formed thereon by bonding abrasive grainswith a metal bond. Chamfering of the base periphery is advantageous inthat even when the blade is over-ground in error beyond the boundarybetween the base and the abrasive layer during grinding for bladethickness adjustment purpose, the metal bond is left at the boundary toprevent the blade section from being separated apart. The angle andquantity of chamfer may be determined in accordance with the thicknessof the base and the average grain size of abrasive grains because therange available for chamfering depends on the thickness of the base.

The abrasive grains used herein are diamond grains and/or CBN grains.The abrasive grains should have been coated with a magnetic material.The size and hardness of abrasive grains prior to the magnetic materialcoating are determined in accordance with the intended application.

For example, diamond grains (including natural diamond and industrialsynthetic diamond) or cubic boron nitride (CBN) grains may be usedalone. A mixture of diamond grains and CBN grains is also acceptable.Depending on the workpiece, abrasive grains of each type may be selectedfrom single crystal grains and polycrystalline grains and used alone orin admixture for adjusting fragility. Further, sputtering a metal suchas Fe, Co or Cr onto surfaces of abrasive grains to a thickness of about1 μm is effective for enhancing the bond strength to the magneticmaterial to be subsequently coated.

Preferably abrasive grains have an average grain size of 10 to 300 μmalthough the grain size depends on the thickness of the base. If theaverage grain size is less than 10 μm, there may be left smaller voidsbetween abrasive grains, allowing problems like glazing and loading tooccur during the cutting operation and losing the cutting ability. Ifthe average grain size is more than 300 μm, problems may arise, forexample, magnet pieces cut thereby may have rough surfaces. With thecutting efficiency and lifetime taken into account, abrasive grains of acertain size within the range may be used alone or as a mixture ofgrains of different sizes.

The abrasive grains are previously coated with a magnetic material suchthat the coated abrasive grains may be magnetically attracted in a shorttime even to a base of low saturation magnetization cemented carbide andfixedly held thereto to prevent shedding during the bonding step byplating. Specifically, the coated abrasive grains may have a massmagnetic susceptibility χg of preferably at least 0.2, more preferablyat least 0.39. The magnetic material is typically at least one metalselected from Ni, Fe, and Co, an alloy of two or more such metals, or analloy of one such metal or alloy with at least one metal selected from Pand Mn. The abrasive grains are coated with such a magnetic material byany well-known technique such as sputtering, electroplating orelectroless plating until the thickness of the coating reaches 0.5 to100%, preferably 2 to 80% of the diameter of abrasive grains.

Since the magnetic susceptibility of coated abrasive grains depends onthe magnetic susceptibility of the coating magnetic material and thethickness of the magnetic material coating, the type of magneticmaterial should be selected so as to gain a necessary attraction forcefor abrasive grains of a certain size. Nevertheless, even an electrolessplated nickel-phosphorus coating having a low magnetic susceptibilitydue to a high phosphorus content can be increased in susceptibility to acertain extent by heat treatment. Also a multilayer coating of layershaving different susceptibility is possible, for example, a coatingincluding a layer having a low susceptibility and an overlying layerhaving a high susceptibility. Thus, the magnetic susceptibility ofcoated abrasive grains may be tailored in accordance with a particularsituation.

As long as the coated abrasive grains have a mass magneticsusceptibility χg of at least 0.2, preferably at least 0.39, the coatedabrasive grains are quickly magnetized by a magnetic field which isproduced near the periphery of the base. Then the abrasive grains aremagnetically attracted substantially equally at any sites within a space64 defined by the base and permanent magnet holders of a jig as shown inFIG. 3. If the mass magnetic susceptibility χg of the coated abrasivegrains is less than 0.2, the abrasive grains within that space may notbe fully attracted. With such weak attraction, some abrasive grains mayfall off during plating, failing to form an abrasive grain layer (orblade section) or forming an abrasive grain layer which is porous andthus low in mechanical strength.

The mass magnetic susceptibility χg of abrasive grains may be determinedby providing a resinous vessel having an outer diameter of 8 mm, aninner diameter of 6 mm, and a height of 5 mm, distributing grainsuniformly and thinly so as to form one or two layers of grains in thevessel, taking the grains out of the vessel, measuring the weight of thegrains, returning them into the vessel, placing a paraffin with amelting point of about 50° C. on the grain layer, and heating the vesselin an oven at 60° C. Once the paraffin is melted, the vessel is closedwith a lid and cooled. An initial magnetization curve (4πI-H) of thesample is measured at a temperature of 24-25° C. by means of a vibratingsample magnetometer (VSM). A gradient at the inflection point of theinitial magnetization curve gives a differential susceptibility, whichis divided by the sample weight, yielding the mass magneticsusceptibility χg of abrasive grains. Notably, the magnetic field iscalibrated using a standard Ni sample, and the density of abrasivegrains is measured as a tap bulk density.

The thickness of magnetic material coating should fall in an appropriaterange because the coating thickness can affect the size of voids createdduring formation of the blade section. The minimum thickness of coatingis preferably 2.5 μm that is a thickness at which overall abrasivegrains can be coated by plating without substantial voids. For example,for abrasive grains with an average grain size of 300 μm that is themaximum of the preferred average grain size range, the coating thicknessmay be at least 0.5%, more preferably at least 0.8% of the grain size.As long as the coating of magnetic material has a thickness in therange, it offers a retaining force capable of reducing shedding ofabrasive grains when the outer blade cutting wheel is used in cuttingoperation. As long as a magnetic material of proper type is selected forcoating, abrasive grains are attracted and held to or near the outerperiphery of the base by the magnetic field during the plating step,without falling off.

For abrasive grains with an average grain size of 10 μm that is theminimum of the preferred average grain size range, the maximum coatingthickness is preferably up to 100% of the average grain size of abrasivegrains, because otherwise a fraction of abrasive grains not effectivelyfunctioning during cutting operation increases, a portion of preventingself-sharpening of abrasive grains increases, and the machining abilitydegrades.

The metal bond for bonding abrasive grains together is a plating metalor alloy. When a blade section is to be formed, a permanent magnet mustbe disposed near the outer periphery of the base to produce a magneticfield. For example, two or more permanent magnets having a remanence (orresidual magnetic flux density) of at least 0.3 T are disposed on theside surfaces of the base positioned inside the outer periphery thereofor within spaces disposed inside the outer periphery of the base andspaced a distance of not more than 20 mm from the side surfaces of thebase, to thereby produce a magnetic field of at least 8 kA/m in a spaceextending a distance of 10 mm or less from the outer periphery of thebase. The magnetic field acts on the diamond and/or CBN abrasive grainspre-coated with a magnetic material, to produce a magnetic attractionforce. By this magnetic attraction force, the abrasive grains aremagnetically attracted and fixedly held to or near the base outerperiphery. With the abrasive grains held fixedly, electroplating orelectroless plating of a metal or alloy is carried out on the base outerperiphery for thereby bonding the abrasive grains to the base outerperiphery.

The jig used in this process comprises a pair of holders each comprisinga cover of insulating material having a greater outer diameter than theouter diameter of the base and a permanent magnet disposed on andfixedly secured to the cover inside the base outer periphery. Platingmay be carried out while the base is held between the holders.

Referring to FIGS. 2 and 3, one exemplary jig for use in the platingprocess is shown. The jig comprises a pair of holders 50, 50 eachcomprising a cover 52 of insulating material and a permanent magnet 54mounted on the cover 52. A base 1 is sandwiched between the holders 50and 50. The permanent magnet 54 is preferably buried in the cover 52.Alternatively, the permanent magnet 54 is mounted on the cover 52 sothat the magnet 54 may be in abutment with the base 1 when assembled.

The permanent magnet built in the jig should have a magnetic forcesufficient to keep abrasive grains attracted to the base during theplating process of depositing a metal bond to bond abrasive grains.Although the necessary magnetic force depends on the distance betweenthe base outer periphery and the magnet, and the magnetization andsusceptibility of a magnetic material coated on abrasive grains, adesired magnetic force may be obtained from a permanent magnet having aremanence of at least 0.3 T and a coercivity of at least 0.2 MA/m,preferably a remanence of at least 0.6 T and a coercivity of at least0.8 MA/m, and more preferably a remanence of at least 1.0 T and acoercivity of at least 1.0 MA/m.

The greater remanence a permanent magnet has, the greater gradient themagnetic field produced thereby has. Thus a permanent magnet with agreater remanence value is convenient when it is desired to locallyattract abrasive grains. In this sense, use of a permanent magnet havinga remanence of at least 0.3 T is preferred for preventing abrasivegrains from separating apart from the base due to agitation of a platingsolution and vibration by rocking motion of the base-holding jig duringthe plating process.

As the coercivity is greater, the magnet provides a stronger magneticattraction of abrasive grains to the base for a long period even whenexposed to a high-temperature plating solution. Then the freedom ofchoice with respect to the position, shape and size of a magnet used isincreased, facilitating the manufacture of the jig. A magnet having ahigher coercivity is selected from those magnets meeting the necessaryremanence.

In view of potential contact of the magnet with plating solution, thepermanent magnet is preferably coated so that the magnet may be morecorrosion resistant. The coating material is selected under suchconditions as to minimize the dissolution of the coating material in theplating solution and the substitution for metal species in the platingsolution. In an embodiment wherein a metal bond is deposited from anickel plating bath, the preferred coating material for the magnet is ametal such as Cu, Sn or Ni or a resin such as epoxy resin or acrylicresin.

The shape, size and number of permanent magnets built in the jig dependon the size of the cemented carbide base, and the position, directionand strength of the desired magnetic field. For example, when it isdesired to uniformly bond abrasive grains to the base outer periphery, amagnet ring corresponding to the outer diameter of the base may bedisposed, or arc shaped magnet segments corresponding to the outerdiameter of the base or rectangular parallelepiped magnet segmentshaving a side of several millimeters long may be continuously andclosely arranged along the base outer periphery. For the purpose ofreducing the cost of magnet, magnet segments may be spaced apart toreduce the number of magnet segments.

The spacing between magnet segments may be increased, though dependingon the remanence of magnet segments used. With magnet segments spacedapart, magnetic material-coated abrasive grains are divided into onegroup of grains attracted and another group of grains not attracted.Then abrasive grains are alternately bonded to some areas, but not toother areas of the base outer periphery. A blade section consisting ofspaced segments is formed.

With respect to the magnetic field produced near the base outerperiphery, a variety of magnetic fields can be produced by changing acombination of the position and magnetization direction of permanentmagnets mounted to two holders sandwiching the base. By repeatingmagnetic field analysis and experiments, the arrangement of magnets isdetermined so as to produce a magnetic field of at least 8 kA/m,preferably at least 40 kA/m within a space extending a distance of 10 mmor less from the outer periphery of the base. When the strength of themagnetic field is less than 8 kA/m, it has a short magnetic force toattract magnetic material-coated abrasive grains, and if plating iscarried out in this state, abrasive grains may be moved away during theplating process, and as a consequence, a blade section having many voidsis formed, or abrasive grains are bonded in a dendritic way, resultingin a blade section having a size greater than the desired. Subsequentdressing may cause the blade section to be separated apart or take alonger time. These concerns may increase the cost of manufacture.

Preferably the permanent magnet is placed nearer to the portion to whichabrasive grains are attracted. Generally speaking, the permanent magnetis placed on the side surface of the base inside the outer peripherythereof or within a space situated inside the outer periphery of thebase and extending a distance of not more than 20 mm from the sidesurface of the base and preferably within a space situated inside theouter periphery and extending a distance of not more than 10 mm from theside surface of the base. At least two permanent magnets having aremanence of at least 0.3 T (specifically at least one magnet perholder) are placed at specific positions within the spaces such that themagnets are entirely or partially situated within the spaces whereby amagnetic field having a strength of at least 8 kA/m can be producedwithin a space extending a distance of not more than 10 mm from theouter periphery of the base. Then, independent of whether the base ismade of a material having a high saturation magnetization and alikelihood to induce a magnetic force such as alloy tool steel orhigh-speed steel, or a material having a low saturation magnetizationand a less likelihood to induce a magnetic force such as cementedcarbide, a magnetic field having an appropriate magnetic force can beproduced near the outer periphery of the base. When magneticmaterial-coated abrasive grains are fed in the magnetic field, thecoating is magnetized and consequently, the abrasive grains areattracted and held to or near the outer periphery of the base.

With respect to the position of the magnet relative to the outerperiphery of the base, if the magnet is not placed within the spacedefined above, specifically if the magnet is placed outside the outerperiphery of the base, though close thereto, for example, at a distanceof 0.5 mm outward of the outer periphery of the base, then the magneticfield strength near the outer periphery of the base is high, but aregion where the magnetic field gradient is reversed is likely to exist.Then abrasive grains tend to show a behavior of emerging upward from thebase and shedding away. If the position of the magnet is inside theouter periphery of the base, but at a distance of more than 20 mm fromthe outer periphery of the base, then the magnetic field in the spaceextending a distance of not more than 10 mm from the outer periphery ofthe base tends to have a strength of less than 8 kA/m, with a risk ofthe force of magnetically attracting abrasive grains becoming short. Insuch a case, the strength of the magnetic field may be increased byenlarging the size of magnet. However, a large sized magnet produces amagnetic field of increased strength not only near the site to whichabrasive grains are attracted, but over the surrounding, which isundesirable because some abrasive grains can be attached to the site towhich abrasive grains are not to be attracted. A large sized magnet isnot so practical because the magnet-built-in jig also becomes large.

The shape of the jig (holders) conforms to the shape of the base. Thesize of the jig (holders) is such that when the base is sandwichedbetween holders, the permanent magnet in the holder may be at thedesired position relative to the base. For a base having an outerdiameter of 125 mm and a thickness of 0.26 mm and an array of permanentmagnet segments of 2.5 mm long by 2 mm wide by 1.5 mm thick, forexample, a disc having an outer diameter of at least 125 mm and athickness of about 20 mm is used as the holder.

Specifically, the outer diameter of the jig or holder is selected to beequal to or greater than {the outer diameter of the base plus (height ofabrasive layer) multiplied by 2}, so as to ensure a height or radialprotrusion (H2 in FIG. 1C) of the abrasive grain layer, and thethickness of the jig or holder is selected so as to provide a strengthsufficient to prevent warpage due to abrupt temperature changes bymoving into and out of a hot plating bath. The thickness of the portionof the holder which comes in contact with abrasive grains may be reducedthan the remaining portion so as to ensure an axial protrusion (T3 inFIG. 1C) of the abrasive grain layer in the thickness direction of thebase. A masking tape having a thickness equal to the axial protrusionmay be attached to that portion so that the thickness may become equalto that of the remaining portion.

The material of which the jig or holders are made is preferably aninsulating material on which no plating deposits, because the overalljig having the base sandwiched between the holders is immersed in a hotplating bath for depositing a metal bond on the base. More desirably theinsulating material should have chemical resistance, heat resistance upto about 90° C., and thermal shock resistance sufficient to maintain thesize constant even when exposed to repeated rapid thermal cycling inmoving into and out of the plating bath. Also desirably the insulatingmaterial should have dimensional stability sufficient to prevent theholders from being warped by the internal stresses (accumulated duringmolding and working) to create a gap between the holder and the basewhen immersed in a hot plating bath. Of course, the insulating materialshould be so workable that a groove for receiving a permanent magnet atan arbitrary position may be machined at a high accuracy withoutfissures or chips.

Specifically, the holders may be made of engineering plastics such asPPS, PEEK, POM, PAR, PSF and PES and ceramics such as alumina. A holderis prepared by selecting a suitable material, determining a thicknessand other dimensions in consideration of mechanical strength, moldingthe material to the dimensions, and machining a groove for receiving apermanent magnet and a recess for receiving an electric supply electrodewhich is necessary when electroplating is carried out. On use, a pair ofsuch holders thus prepared is assembled so as to sandwich the basetherebetween. When the holders are assembled together with an electrodefor electric supply to the base to enable electroplating, thisassembling procedure affords both electric supply and mechanicalfastening and leads to a compact assembly as a whole. It is, of course,preferred that a plurality of jigs be connected as shown in FIG. 2 sothat a plurality of bases may be plated at a time, because theproduction process becomes more efficient.

Specifically, as shown in FIG. 2, a cathode 56 which serves forelectroplating and as a base retainer is fitted in a central recess inthe cover 52. A jig is assembled by combining a pair of holders 50 witha base 1, inserting a conductive support shaft 58 into the bores of theholders and base, and fastening them together. In the assembled state,the cathodes 56 are in contact with the shaft 58, allowing for electricsupply from the shaft 58 to the cathodes 56. In FIG. 2, two jigs eachconsisting of a pair of holders 50, 50 are mounted on the shaft 58 at asuitable spacing, using a spacer 60 and an end cap 62. Understandablythe jig shown in FIG. 2 is intended for electroplating. In the case ofelectroless plating, the cathode is not necessary, a non-conductiveretainer may be used instead, and the support shaft need not necessarilybe conductive.

Using the jig, plating is carried out as follows. The jig is assembledby sandwiching the base 1 between the permanent magnet-built-in holders50, 50. In this state, as shown in FIG. 3, a space 64 is defined byperipheral portions 52 a, 52 a (extending outward beyond the base) ofcovers 52, 52 of holders 50, 50 and the outer periphery of the base 1. Asuitable amount of abrasive grains pre-coated with a magnetic materialis weighed by a balance and fed into the space 64 where the abrasivegrains are magnetically attracted and held.

The amount of abrasive grains held in the space depends on the outerdiameter and thickness of the base, the size of abrasive grains, and thedesired height and width of the blade section to be formed. Alsopreferably the process of holding abrasive grains and effecting platingis repeated plural times so that the amount of abrasive grains per unitvolume may be equalized at any positions on the base outer periphery andabrasive grains may be tenaciously bonded by the plating technique.

In this way, a blade section is formed. The blade section preferablycontains abrasive grains in a volume fraction of 10 to 80% by volume,and more preferably 30 to 75% by volume. A fraction of less than 10% byvolume means that less abrasive grains contribute to cutting, leading toincreased resistance during the cutting operation. A fraction in excessof 80% by volume means that the deformation amount of cutting edgeduring the cutting operation is reduced, leaving cut marks on the cutsurface and aggravating the dimensional accuracy and appearance of cutpieces. For these reasons, the cutting speed must be slowed down. It isthus preferred to adjust the volume fraction of abrasive grains for aparticular application by changing the thickness of the magneticmaterial coating on abrasive grains to change the grain size.

As shown in FIG. 1C, the blade section 20 consists of a pair of clamplegs 22 a, 22 b which clamp the outer rim of the base 10 therebetween inan axial direction and a body (20) which extends radially outward beyondthe outer rim (periphery) of the base 10. It is noted that this divisionis for convenience of description because the legs and the body areintegral to form the blade section. The thickness of the blade section20 is greater than the thickness of the base 10. To form the bladesection of this design, the space 64 is preferably configured as shownin FIG. 3.

More specifically, the clamp legs 22 a, 22 b of the blade section 20which clamp the outer rim of the base 10 therebetween each preferablyhave a length H1 of 0.1 to 10 mm, and more preferably 0.5 to 5 mm. Thelegs 22 a, 22 b each preferably have a thickness T3 of at least 5 μm(=0.005 mm), more preferably 5 to 2,000 μm, and even more preferably 10to 1,000 μm. Then the total thickness of legs 22 a, 22 b is preferablyat least 0.01 mm, more preferably 0.01 to 4 mm, and even more preferably0.02 to 2 mm. The blade section 20 is thicker than the base 10 by thistotal thickness. If the length H1 of clamp legs 22 a, 22 b is less than0.1 mm, they are still effective for preventing the rim of the cementedcarbide base from being chipped or cracked, but less effective forreinforcing the base and sometimes fail to prevent the base from beingdeformed by the cutting resistance. If the length H1 exceeds 10 mm,reinforcement of the base is made at the sacrifice of expense. If thethickness T3 of clamp leg is less than 5 μm, such thin legs may fail toenhance the mechanical strength of the base or to effectively dischargethe swarf sludge.

As shown in FIGS. 4A to 4D, the clamp legs 22 a, 22 b may consist of ametal bond 24 and abrasive grains 26 (FIG. 4A), consist of metal bond 24(FIG. 4B), or include an underlying layer consisting of metal bond 24covering the base 10 and an overlying layer consisting of metal bond 24and abrasive grains 26 (FIG. 4C). Notably the strength of the bladesection may be further increased by depositing a metal bond on thestructure of FIG. 4C so as to surround the overall outer surface asshown in FIG. 4D.

In the embodiments shown in FIGS. 4B to 4D, the clamp leg inner portionsin contact with the base 10 are formed solely of metal bond 24. To thisend, the base is masked so that only the portions of the base on whichthe clamp legs are to be formed are exposed, and plating is carried outon the unmasked base portions. This may be followed by mounting the basein the jig, charging the space 64 with abrasive grains 26, and effectingplating. After the electroplating of abrasive grains, the base 10 may bemasked with another pair of holders 50, 50 having a smaller outerdiameter such that the electroplated portion is exposed, and plating iscarried out again, forming a layer consisting of metal bond 24 as theblade section outermost layer as shown in FIG. 4D.

Referring back to FIG. 1C, the body of the blade section 20 whichextends radially outward beyond the periphery of the base 10 has alength H2 which is preferably 0.1 to 10 mm, and more preferably 0.3 to 8mm, though may vary with the size of abrasive grains to be bonded. Ifthe body length H2 is less than 0.1 mm, the blade section may beconsumed within a short time by impacts and wears during the cuttingoperation, which indicates a cutting wheel with a short lifetime. If thebody length H2 exceeds 10 mm, the blade section may become susceptibleto deformation, though dependent on the blade thickness (T2 in FIG. 1C),resulting in cut magnet pieces with wavy cut surfaces and hence,worsening dimensional accuracy. The body of the blade section consistsessentially of abrasive grains 26, metal bond 24, and resin binder.

The metal bond is a metal or alloy deposited by plating. The metal bondused herein is at least one metal selected from the group consisting ofNi, Fe, Co, Cu, and Sn, an alloy consisting of at least two of theforegoing metals, or an alloy consisting of at least one of theforegoing metals or alloys and one or both of phosphorus (P) andmanganese (Mn). The metal or alloy is deposited by plating so as to forminterconnects between abrasive grains and between abrasive grains andthe base.

The method of depositing the metal bond by plating is generallyclassified into two, an electroplating method and an electroless platingmethod. In the practice of the invention, the electroplating methodwhich is easy to control internal stresses remaining in the metal bondand low in production cost and the electroless (or chemical) platingmethod which ensures relatively uniform deposition of metal bond as longas the plating solution penetrates there may be used alone or incombination so that the blade section may contain voids in anappropriate range to be described later.

The stress in the plating film may be controlled by suitable means. Forexample, in single metal plating such as copper or nickel plating,typically nickel sulfamate plating, the stress may be controlled byselecting the concentration of the active ingredient or nickelsulfamate, the current density during plating, and the temperature ofthe plating bath in appropriate ranges, and adding an organic additivesuch as o-benzenesulfonimide or p-toluenesulfonamide, or an element suchas Zn, S or Mn. Besides, in alloy plating such as Ni—Fe alloy, Ni—Mnalloy, Ni—P alloy, Ni—Co alloy or Ni—Sn alloy, the stress may becontrolled by selecting the content of Fe, Mn, P, Co or Sn in the alloy,the temperature of the plating bath, and other parameters in appropriateranges. In the case of alloy plating, addition of organic additives may,of course, be effective for stress control.

Plating may be carried out in a standard way by selecting any one ofwell-known plating baths for deposition of a single metal or alloy andusing plating conditions common to that bath.

Examples of the preferred electroplating bath include a sulfamate Wattsnickel electroplating bath containing 250 to 600 g/L of nickelsulfamate, 50 to 200 g/L of nickel sulfate, 5 to 70 g/L of nickelchloride, 20 to 40 g/L of boric acid, and an amount ofo-benzenesulfonimide; and a pyrophosphoric acid copper electroplatingbath containing 30 to 150 g/L of copper pyrophosphate, 100 to 450 g/L ofpotassium pyrophosphate, 1 to 20 mL/L of 20% ammonia water, and 5 to 20g/L of potassium nitrate. A typical electroless plating bath is anickel-phosphorus alloy electroless plating bath containing 10 to 50 g/Lof nickel sulfate, 10 to 50 g/L of sodium hypophosphite, 10 to 30 g/L ofsodium acetate, 5 to 30 g/L of sodium citrate, and an amount ofthiourea.

By the plating method, abrasive grains which may be diamond abrasivegrains, CBN abrasive grains or a mixture of diamond and CBN abrasivegrains are bonded together and to the outer periphery of the base toform at a high accuracy a blade section having dimensions approximate tothe final shape.

The blade section thus formed contains voids between abrasive grains andbetween abrasive grains and the base. According to the invention, athermoplastic resin having a melting point of up to 350° C. isinfiltrated into the voids and solidified thereat, or a liquidthermosetting resin composition having a curing temperature of up to350° C. is infiltrated into the voids and cured thereat. Therefore, theblade section of the outer blade cutting wheel is characterized in thata thermoplastic resin having a melting point of up to 350° C. or athermoset resin resulting from a liquid thermosetting resin compositionhaving a curing temperature of up to 350° C. is present between abrasivegrains and between abrasive grains and the base throughout the bladesection from the surface to the interior.

Suitable infiltrating resins include thermoplastic resins andthermosetting resins, typically epoxy resins, acrylic resins, phenolicresins, polyamide resins, polyimide resins, and modified resins of theforegoing, which may be used alone or in admixture.

The thermoplastic or thermosetting resin may be infiltrated into theblade section, for example, by working the thermoplastic resin into awire with a diameter of 0.1 to 2.0 mm, preferably 0.8 to 1.5 mm,particles, or a thin-film ring of the same shape and size as the bladesection having a thickness of 0.05 to 1.5 mm, resting the wire,particles or ring on the blade section, heating the blade section on aheater such as a hot plate or in an oven to a temperature above themelting point, holding the temperature for letting the molten resininfiltrate into the blade section, and thereafter slowly cooling to roomtemperature. A thermosetting resin may be infiltrated by blending theresin with an organic solvent, a curing agent and the like to form aliquid thermosetting resin composition, casting the composition on theblade section, letting the composition infiltrate into the bladesection, heating at or above the curing temperature, thereby curing, andthereafter slowly cooling to room temperature. Alternatively,infiltration is carried out by placing the outer blade cutting wheel ina lower mold half with a clearance near the blade section, charging themold half with a weighed amount of the resin or resin composition,mating an upper mold half with the lower mold half, heating the matedmold while applying a certain pressure across the mold, for letting theresin or resin composition infiltrate into the blade section. Thereafterthe mold is cooled, the pressure is then released, and the wheel istaken out of the mold. The cooling step following heating should be slowso as to avoid any residual strains.

When a resin having relatively good wettability is used, infiltrationmay be carried out by sandwiching the base between metal members ofstainless steel, iron or copper, conducting electricity to the metalmembers, causing the metal members to generate heat, thereby heating thebase and the blade section, and bringing the heated blade section incontact with a molten resin or liquid resin composition.

In the resulting blade section, the abrasive grains, the magneticmaterial covering abrasive grains, the metal bond, and the resininfiltrated into voids are properly dispersed.

The resin to be infiltrated into the blade section should preferablyhave the following physical properties. The melting point is preferablynot higher than 350° C. In the case of thermoplastic resin, the meltingpoint is not higher than 350° C., preferably not higher than 300° C.,for the purpose of preventing the cemented carbide base from beingdistorted to aggravate dimensional accuracy or change mechanicalstrength, and preventing the blade section from deformation or straingeneration due to an outstanding difference in thermal expansion betweenthe cemented carbide base and the blade section. In the case ofthermosetting resin, the melting temperature is preferably at least 10°C. because it suffices that a thermosetting resin composition has asufficient fluidity to infiltrate at room temperature.

The resin preferably has an elasticity as demonstrated by a Poisson'sratio between 0.3 and 0.48, more preferably between 0.33 and 0.44. Aresin with a Poisson's ratio of less than 0.3 lacks flexibility and isdifficult to smoothly merge the cut surfaces. A resin with a Poisson'sratio of more than 0.48 is short of other physical properties such ashardness, with a risk of the blade edge experiencing noticeabledeformation. The Poisson's ratio may be measured by the pulse ultrasonicmethod using an infiltrating resin sample of 15×15×15 mm.

The resin may have a hardness which is not so high as to preventself-sharpening of abrasive grains (a phenomenon that new abrasivegrains emerge, contributing to the cutting operation) when abrasivegrains are worn, broken or shed during the cutting operation, and whichis lower than that of the metal bond for bonding the abrasive grains andthe magnetic material coating thereon. Also preferably, the resin shouldnot undergo strength changes or corrosion even when exposed to themachining fluid or coolant used during the machining process.

If necessary, the blade section having the resin infiltrated therein istailored to the desired size by grinding with a grinding wheel ofaluminum oxide, silicon carbide or diamond or electro-dischargemachining. At this point, the blade section at the edge may be chamfered(beveled or rounded) to a degree of at least C0.1 or R0.1, thoughdepending on the thickness of the blade section, because such chamferingis effective for reducing cut marks on the cut surface or mitigatingchipping of a magnet piece at the end.

On use of the outer blade cutting wheel of the invention, variousworkpieces may be cut thereby. Typical workpieces include R—Co rareearth sintered magnets and R—Fe—B rare earth sintered magnets wherein Ris at least one of rare earth elements inclusive of Y. These magnets areprepared as follows.

R—Co rare earth sintered magnets include RCo₅ and R₂Co₁₁ systems. Ofthese, the R₂Co₁₇ magnets have a composition (in % by weight) comprising20-28% R, 5-30% Fe, 3-10% Cu, 1-5% Zr, and the balance of Co. They areprepared by weighing source materials in such a formulation, meltingthem, casting the melt, and finely pulverizing the alloy to an averageparticle size of 1-20 μm, yielding a R₂Co₁₇ magnet powder. The powder isthen compacted in a magnetic field and sintered at 1,100-1,250° C. for0.5-5 hours. The sintered body is subjected to solution treatment at atemperature lower than the sintering temperature by 0-50° C. for 0.5-5hours, and aging treatment of holding at 700-950° C. for a certain timeand subsequent cooling.

R—Fe—B rare earth sintered magnets have a composition (in % by weight)comprising 5-40% R, 50-90% Fe, and 0.2-8% B. An additive element orelements may be added thereto for improving magnetic properties andcorrosion resistance, the additive elements being selected from C, Al,Si, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Sn, Hf, Ta, W,etc. The amount of additive element is up to 30% by weight for Co, andup to 8% by weight for the other elements. The magnets are prepared byweighing source materials in such a formulation, melting them, castingthe melt, and finely pulverizing the alloy to an average particle sizeof 1-20 μm, yielding a R—Fe—B magnet powder. The powder is thencompacted in a magnetic field and sintered at 1,000-1,200° C. for 0.5-5hours, followed by aging treatment of holding at 400-1,000° C. for acertain time and subsequent cooling.

The outer blade cutting wheel of the invention is used to cut a rareearth magnet block into magnet pieces at a high dimensional accuracywithout leaving cut marks on the cut surface. This is true particularlywhen the blade section at the edge has a compressive shearing stress ina specific range. For example, the outer blade cutting wheel includes ablade section which has a thickness of 0.1 to 1.0 mm and an outerdiameter of 80 to 200 mm, and is chamfered (rounded or beveled) at theedge to a degree of at least R0.1 or C0.1. The cutting wheel ishorizontally mounted on a jig by sandwiching the cutting wheel betweencircular iron plates of 5 mm thick such that only the blade section isexposed. The wheel is thus held so that the base may not be warped uponcompression. At a position spaced 0.3 mm outward from the outerperiphery of the base, the blade section is compressed by a probe in anaxial direction of the rotating shaft of the cutting wheel (or thicknessdirection of the blade section) at a linear speed of 1 mm/min, the probehaving a contact area with a length equal to a radial protrusion (mm) ofthe blade section minus 0.3 mm and a width of 10 mm. The stress reactingto the travel distance of the probe is measured. Compression iscontinued until the blade section is ruptured. As the travel distance ofthe probe increases, a region where the graph exhibits linearity, thatis, a region where the stress is in proportion to the travel distance ofthe probe is acknowledged. A gradient of the graph in the region ofstress proportional to deformation amount is computed. As long as thegradient is in a range of 100 to 10,000 N/mm, the cutting wheel iseffective in cutting a magnet block into magnet pieces at a highdimensional accuracy without cut marks on the cut surface.

EXAMPLES

Examples and Comparative Example are given below by way of illustrationand not by way of limitation.

Example 1

A cemented carbide consisting of 90 wt % WC and 10 wt % Co was machinedinto an annular thin disc having an outer diameter of 125 mm, an innerdiameter of 40 mm, and a thickness of 0.3 mm, which served as a base.The base had a Young's modulus of 600 GPa and a saturation magnetizationof 127 kA/m (0.16 T).

The cemented carbide base was masked with adhesive tape so that only acircumferential region of either surface extending 1.0 mm inward fromthe outer periphery was exposed. The base was immersed in a commerciallyavailable aqueous alkaline solution at 40° C. for 10 minutes fordegreasing, washed with water, and immersed in an aqueous solution of30-80 g/L of sodium pyrophosphate at 50° C. where electrolysis waseffected at a current density of 2-8 A/dm². The base was ultrasonicwashed in deionized water and immersed in a sulfamate Watts nickelplating bath at 50° C. where an undercoat was plated at a currentdensity of 5-20 A/dm². Once the masking tape was peeled off, the basewas washed with water.

A polyphenylene sulfide (PPS) resin disc having an outer diameter of 130mm and a thickness of 10 mm was machined on one side surface to form agroove having an outer diameter of 123 mm, an inner diameter of 119 mm,and a depth of 1.5 mm. In the groove of the disc, 75 permanent magnetsegments of 2.5 mm long by 2 mm wide by 1.5 mm thick (N39UH by Shin-EtsuRare Earth Magnets Co., Ltd., Br=1.25 T) were arranged at an equalspacing, with the thickness direction of the segment aligned with thedepth direction of the groove. The groove was filled with an epoxy resinto fixedly secure the magnet segments in the groove, completing amagnet-built-in holder. The base was sandwiched between a pair of suchholders to construct a jig, with the magnet sides of the holders facedinside. In the sandwiched state, the magnet was spaced inward a distanceof 1 mm from the base outer periphery along the base surface. The magnetproduced a magnetic field near the base outer periphery, which wasanalyzed to have a strength of at least 8 kA/m (0.01 T) within a spaceextending a distance of 10 mm from the base outer periphery.

Diamond abrasive grains were previously NiP-plated to form coateddiamond abrasive grains having a mass magnetic susceptibility χg of0.588 and an average grain size of 135 μm. In a recess defined by theholders and the base, 0.4 g of the coated diamond abrasive grains werefed whereby the abrasive grains were magnetically attracted to anduniformly distributed over the entire base outer periphery. The jig withthe abrasive grains attracted thereto was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electroplating was effected at acurrent density of 5-20 A/dm². The jig was taken out and washed withwater. The procedure of magnetically attracting 0.4 g of coated diamondabrasive grains, electroplating, and water washing was repeated.

The holders of the jig were replaced by PPS resin disc holders having anouter diameter of 123 mm and a thickness of 10 mm. The base wassandwiched between the holders so that the side surfaces of the abrasivegrain layer were exposed. The jig was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electricity was conducted at acurrent density of 5-20 A/dm² to deposit a plating over the entire bladesection. The jig was taken out and washed with water, after which thebase was dismounted and dried, obtaining an outer blade cutting wheel.

A liquid epoxy resin composition obtained by dissolving bisphenol Adiglycidyl ether and dicyandiamide as main resin-forming components inan organic solvent was coated onto the side surface of the blade sectionof the outer blade cutting wheel and held for 3 minutes. The wheel wasplaced in an oven at 180° C. where it was held for about 120 minutes.The oven was turned off whereupon the wheel was allowed to cool down inthe oven. It is noted that the cured epoxy resin had a Poisson's ratioof 0.34. FIG. 5 is a photomicrograph of the side surface of the bladesection.

Using a tool grinding machine, the wheel was ground such that theabrasive layer protruded a distance (T3) of 50 μm beyond the cementedcarbide base on each surface. After the protrusion or thickness andouter diameter of the abrasive layer were tailored, the wheel wasdressed, yielding a cemented carbide base outer blade cutting wheelhaving an abrasive grain layer or blade section with a thickness (T2) of0.4 mm and an outer diameter of 127 mm.

Example 2

A cemented carbide consisting of 90 wt % WC and 10 wt % Co was machinedinto an annular thin disc having an outer diameter of 125 mm, an innerdiameter of 40 mm, and a thickness of 0.3 mm, which served as a base.

The cemented carbide base was masked with adhesive tape so that only acircumferential region of either surface extending 1.5 mm inward fromthe outer periphery was exposed. The base was immersed in a commerciallyavailable aqueous alkaline solution at 40° C. for 10 minutes fordegreasing, washed with water, and immersed in an aqueous solution of30-80 g/L of sodium pyrophosphate at 50° C. where electrolysis waseffected at a current density of 2-8 A/dm². The base was ultrasonicwashed in deionized water and immersed in a sulfamate Watts nickelplating bath at 50° C. where an undercoat was plated at a currentdensity of 5-20 A/dm². Once the masking tape was peeled off, the basewas washed with water.

A PPS resin disc having an outer diameter of 130 mm and a thickness of10 mm was machined on one side surface to form a groove having an outerdiameter of 123 mm, an inner diameter of 119 mm, and a depth of 1.5 mm.In the groove of the disc, 105 permanent magnet segments of 1.8 mm longby 2 mm wide by 1.5 mm thick (N32Z by Shin-Etsu Rare Earth Magnets Co.,Ltd., Br=1.14 T) were arranged at an equal spacing, with the thicknessdirection of the segment aligned with the depth direction of the groove.The groove was filled with an epoxy resin to fixedly secure the magnetsegments in the groove, completing a magnet-built-in holder. The basewas sandwiched between a pair of such holders to construct a jig, withthe magnet sides of the holders faced inside. In the sandwiched state,the magnet was spaced inward a distance of 1.5 mm from the base outerperiphery along the base surface. The magnet produced a magnetic fieldnear the base outer periphery, which was analyzed to have a strength ofat least 16 kA/m (0.02 T) within a space extending a distance of 10 mmfrom the base outer periphery.

Diamond abrasive grains were previously NiP-plated to form coateddiamond abrasive grains having a mass magnetic susceptibility χg of0.588 and an average grain size of 135 μm. In a recess defined by theholders and the base, 0.4 g of the coated diamond abrasive grains werefed whereby the abrasive grains were magnetically attracted to anduniformly distributed over the entire base outer periphery. The jig withthe abrasive grains attracted thereto was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electroplating was effected at acurrent density of 5-20 A/dm². The jig was taken out and washed withwater. The procedure of magnetically attracting 0.4 g of coated diamondabrasive grains, electroplating, and water washing was repeated threetimes.

The holders of the jig were replaced by PPS resin disc holders having anouter diameter of 123 mm and a thickness of 10 mm. The base wassandwiched between the holders so that the side surfaces of the abrasivegrain layer were exposed. The jig was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electricity was conducted at acurrent density of 5-20 A/dm to deposit a plating over the entire bladesection. The jig was taken out and washed with water, after which thebase was dismounted and dried, obtaining an outer blade cutting wheel.

The liquid epoxy resin composition in Example 1 was coated onto the sidesurface of the blade section of the outer blade cutting wheel and heldfor 5 minutes. The wheel was placed in an oven at 180° C. where it washeld for about 120 minutes. The oven was turned off whereupon the wheelwas allowed to cool down in the oven.

Using a tool grinding machine, the wheel was ground such that theabrasive layer protruded a distance of 50 μm beyond the cemented carbidebase on each surface. After the protrusion or thickness and outerdiameter of the abrasive layer were tailored, the wheel was dressed,yielding a cemented carbide base outer blade cutting wheel having anabrasive grain layer or blade section with a thickness of 0.4 mm and anouter diameter of 129 mm.

Example 3

A cemented carbide consisting of 90 wt % WC and 10 wt % Co was machinedinto an annular thin disc having an outer diameter of 125 mm, an innerdiameter of 40 mm, and a thickness of 0.3 mm, which served as a base.

The cemented carbide base was masked with adhesive tape so that only acircumferential region of either surface extending 1.0 mm inward fromthe outer periphery was exposed. The base was immersed in a commerciallyavailable aqueous alkaline solution at 40° C. for 10 minutes fordegreasing, washed with water, and immersed in an aqueous solution of30-80 g/L of sodium pyrophosphate at 50° C. where electrolysis waseffected at a current density of 2-8 A/dm². The base was ultrasonicwashed in deionized water and immersed in a sulfamate Watts nickelplating bath at 50° C. where an undercoat was plated at a currentdensity of 5-20 A/dm². Once the masking tape was peeled off, the basewas washed with water.

The base was sandwiched between the holders of the jig as in Example 1.Diamond abrasive grains were previously NiP-plated to form coateddiamond abrasive grains having a mass magnetic susceptibility χg of0.392 and an average grain size of 130 μm. In a recess defined by theholders and the base, 0.4 g of the coated diamond abrasive grains werefed whereby the abrasive grains were magnetically attracted to anduniformly distributed over the entire base outer periphery. The jig withthe abrasive grains attracted thereto was immersed in a copperpyrophosphate plating bath at 40° C. where electroplating was effectedat a current density of 1-20 A/dm. The jig was taken out and washed withwater. The wheel was dismounted from the jig and dried.

The liquid epoxy resin composition in Example 1 was coated onto the sidesurface of the blade section of the outer blade cutting wheel and heldfor 5 minutes. The wheel was placed in an oven at 180° C. where it washeld for about 120 minutes. The oven was turned off whereupon the wheelwas allowed to cool down in the oven.

Using a tool grinding machine, the wheel was ground such that theabrasive layer protruded a distance of 50 μm beyond the cemented carbidebase on each surface. After the protrusion or thickness and outerdiameter of the abrasive layer were tailored, the wheel was dressed,yielding a cemented carbide base outer blade cutting wheel having anabrasive grain layer or blade section with a thickness of 0.4 mm and anouter diameter of 126 mm.

Example 4

A cemented carbide consisting of 95 wt % WC and 5 wt % Co was machinedinto an annular thin disc having an outer diameter of 125 mm, an innerdiameter of 40 mm, and a thickness of 0.3 mm, which served as a base.The base had a Young's modulus of 580 GPa and a saturation magnetizationof 40 kA/m (0.05 T).

The cemented carbide base was masked with adhesive tape so that only acircumferential region of either surface extending 1.0 mm inward fromthe outer periphery was exposed. The base was immersed in a commerciallyavailable aqueous alkaline solution at 40° C. for 10 minutes fordegreasing, washed with water, and immersed in an aqueous solution of30-80 g/L of sodium pyrophosphate at 50° C. where electrolysis waseffected at a current density of 2-8 A/dm². The base was ultrasonicwashed in deionized water and immersed in a sulfamate Watts nickelplating bath at 50° C. where an undercoat was plated at a currentdensity of 5-20 A/dm². Once the masking tape was peeled off, the basewas washed with water.

The base was sandwiched between the holders of the jig as in Example 1.Diamond abrasive grains were previously NiP-plated to form coateddiamond abrasive grains having a mass magnetic susceptibility χg of0.392 and an average grain size of 130 μm. In a recess defined by theholders and the base, 0.3 g of the coated diamond abrasive grains werefed whereby the abrasive grains were magnetically attracted to anduniformly distributed over the entire base outer periphery. The jig withthe abrasive grains attracted thereto was immersed in an electrolessnickel-phosphorus alloy plating bath at 80° C. where electroless platingwas effected. The jig was taken out and washed with water. The procedureof magnetically attracting 0.3 g of coated diamond abrasive grains,electroless plating, and water washing was repeated twice. The wheel wasdismounted from the jig and dried.

A liquid acrylic resin composition containing methyl methacrylate,methacrylic acid diester, chlorosulfonated polyethylene, and cumenehydroperoxide was coated onto the side surface of the blade section ofthe outer blade cutting wheel. The wheel was placed in an oven, whichwas moderately evacuated to a vacuum and then heated at 80° C. for 60minutes. The oven was turned off whereupon the wheel was allowed to cooldown in the oven with the vacuum kept. It is noted that the curedacrylic resin had a Poisson's ratio of 0.4.

Using a tool grinding machine, the wheel was ground such that theabrasive layer protruded a distance of 50 μm beyond the cemented carbidebase on each surface. After the protrusion or thickness and outerdiameter of the abrasive layer were tailored, the wheel was dressed,yielding a cemented carbide base outer blade cutting wheel having anabrasive grain layer or blade section with a thickness of 0.4 mm and anouter diameter of 127 mm.

Comparative Example 1

A cemented carbide consisting of 90 wt % WC and 10 wt % Co was machinedinto an annular thin disc having an outer diameter of 125 mm, an innerdiameter of 40 mm, and a thickness of 0.3 mm, which served as a base.

The cemented carbide base was masked with adhesive tape so that only acircumferential region of either surface extending 1.0 mm inward fromthe outer periphery was exposed. The base was immersed in a commerciallyavailable aqueous alkaline solution at 40° C. for 10 minutes fordegreasing, washed with water, and immersed in an aqueous solution of30-80 g/L of sodium pyrophosphate at 50° C. where electrolysis waseffected at a current density of 2-8 A/dm². The base was ultrasonicwashed in deionized water and immersed in a sulfamate Watts nickelplating bath at 50° C. where an undercoat was plated at a currentdensity of 5-20 A/dm². Once the masking tape was peeled off, the basewas washed with water.

The base was sandwiched between the holders of the jig as in Example 1.Diamond abrasive grains were previously NiP-plated to form coateddiamond abrasive grains having a mass magnetic susceptibility χg of0.392 and an average grain size of 130 μm. In a recess defined by theholders and the base, 0.4 g of the coated diamond abrasive grains werefed whereby the abrasive grains were magnetically attracted to anduniformly distributed over the entire base outer periphery. The jig withthe abrasive grains attracted thereto was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electroplating was effected at acurrent density of 5-20 A/dm². The jig was taken out and washed withwater. The procedure of magnetically attracting 0.4 g of coated diamondabrasive grains, electroplating, and water washing was repeated.

The holders of the jig were replaced by PPS resin disc holders having anouter diameter of 123 mm and a thickness of 10 mm. The base wassandwiched between the holders so that the side surfaces of the abrasivegrain layer were exposed. The jig was immersed in a sulfamate Wattsnickel plating bath at 50° C. where electricity was conducted at acurrent density of 5-20 A/dm² to deposit a plating over the entire bladesection. The jig was taken out and washed with water. The base wasdismounted and dried, obtaining an outer blade cutting wheel.

Using a tool grinding machine, the wheel was ground such that theabrasive layer protruded a distance of 50 μm beyond the cemented carbidebase on each surface. After the protrusion or thickness and outerdiameter of the abrasive layer were tailored, the wheel was dressed,yielding a cemented carbide base outer blade cutting wheel having anabrasive grain layer or blade section with a thickness of 0.4 mm and anouter diameter of 127 mm.

Table 1 reports the percent yields of manufacture of the cementedcarbide base outer blade cutting wheels of Examples 1 to 4 andComparative Example 1. A percent plating yield is calculated bypreparing 15 samples in each Example until the step of bonding abrasivegrains by plating, judging samples as good when no abrasive grains shedor the abrasive grain layer was not defective, and dividing the numberof good samples by the number of plated samples. A percent working yieldis calculated by performing the steps following the bond metal platingstep until the dressing step on good samples, judging samples as goodwhen the abrasive grain layer was not defective, and dividing the numberof good samples by the number of starting good plated samples. Anoverall percent yield is the percent plating yield multiplied by thepercent working yield, indicating a percent yield of good samples as thecompleted outer blade cutting wheel relative to the number of startingbases used in the manufacture of cutting wheels.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 4 Example 1Plating 100 100 100 93 100 yield (%) Working 100 100 100 100 87 yield(%) Overall 100 100 100 93 87 yield (%)

It is seen from Table 1 that the yields of Examples are better thanComparative Example 1. In particular, the working yield followingplating is satisfactory. It is demonstrated that the manufacture methodof the invention is improved in productivity as well.

Using the cemented carbide base outer blade cutting wheel, a rare earthsintered magnet block was cutoff machined into magnet pieces. Thecutting accuracy of magnet pieces is plotted in the diagram of FIG. 6.

The cutting accuracy was evaluated by providing ten outer blade cuttingwheels of Examples 1 to 4 and Comparative Example 1, two wheels for eachExample. A multiple wheel assembly was constructed by arranging tencutting wheels at a spacing of 1.5 mm, inserting a rotating shaft intothe bores in the bases, and fastening them together. By operating themultiple wheel assembly at 4,500 rpm and a feed speed of 30 mm/min, aNd—Fe—B rare earth sintered magnet block of 40 mm wide by 130 mm long by20 mm high was cutoff machined into magnet pieces of 40 mm wide by 1.5mm long (=thickness (t)) by 20 mm high. The cutting operation wasrepeated until the number of cut magnet pieces totaled to 1,010. Ofthese, the magnet pieces cut between a pair of cutting wheels of thesame Example were selected for examination. Every size measuring cycleincluded from 41 to #100 pieces, indicating total ten cycles. Early tenpieces in each cycle are sampled out (i.e., #1 to #10 from the firstcycle, #101 to #110 from the second cycle, and so forth, and #1,001 to#1,010 from the last cycle). For ten pieces in each cycle, the thickness(t) of each piece was measured at the center and four corners (fivepoints in total) by a micrometer. A difference between maximum andminimum among five measurements is the cutting accuracy (μm). An averagevalue of the cutting accuracies of ten pieces was computed. This averagevalue of every size measuring cycle is plotted in the diagram of FIG. 6.

In Comparative Example 1, the cutting accuracy worsened after three sizemeasuring cycles (from #301 cut magnet piece et seq.). In Examples 1 to4, the cutting accuracy did not worsen until the tenth cycle (until#1,010 cut magnet piece). It is demonstrated that the outer bladecutting wheels of the invention are fully durable in service.

The elasticity or flexibility of the outer blade cutting wheel wasevaluated, with the results plotted in the diagram of FIG. 7. Thecompressive shearing stress of the outer blade cutting wheel at the edgewas evaluated. Specifically, the outer blade cutting wheel was chamfered(rounded or beveled) at the edge to a degree of at least R0.1 or C0.1.The cutting wheel was horizontally mounted on a jig by sandwiching thecutting wheel between circular iron plates of 5 mm thick such that onlythe blade section was exposed. The wheel was thus held so that the basemight not be warped upon compression. At a position spaced 0.3 mmoutward from the outer periphery of the base, the blade section wascompressed by a probe in an axial direction of the rotating shaft of thecutting wheel (or thickness direction of the blade section) at a linearspeed of 1 mm/min, the probe having a contact area with a length equalto (a protrusion (mm) of the blade section minus 0.3 mm) and a width of10 mm. The stress reacting to the travel distance of the probe wasmeasured by a strength tester AG-1 (Shimadzu Mfg. Co., Ltd.).Compression was continued until the blade section was ruptured.

As seen from FIG. 7, in all examples, as the travel distance of theprobe increased, a region where the graph exhibited linearity, that is,a region where the stress was in proportion to the travel distance ofthe probe was observed. A gradient of the graph in the linear region(i.e., stress/travel distance of probe) was computed, with the resultsshown in Table 2.

TABLE 2 Comparative Example 1 Example 2 Example 3 Example 4 Example 1Gradient 800 300 400 400 18,000 (N/mm)

In the above cutting test, all magnet pieces cut using the outer bladecutting wheels of Examples had good-looking cut surfaces. In the case ofmagnet pieces cut using the outer blade cutting wheels of ComparativeExample 1, some samples had cut marks or steps on the cut surface afterthree size measuring cycles (from #301 cut magnet piece et seq.). It isdemonstrated that as long as the gradient given as stress relative totravel distance of probe and indicative of the elasticity or flexibilityof the cutting wheel is not so high, that is, the blade section has acertain degree of flexibility, the outer blade cutting wheel iseffective for cutoff machining a magnet block into magnet pieces at ahigh dimensional accuracy without cut marks on the cut surface.

It is demonstrated that when a workpiece, typically a rare earthsintered magnet block is cutoff machined into pieces using the outerblade cutting wheels of the invention, the pieces as cut have a highaccuracy without a need for finishing after cutting. Pieces having ahigh dimensional accuracy are available.

1. An outer blade cutting wheel comprising a base in the form of anannular thin disc of cemented carbide having a Young's modulus of 450 to700 GPa, having an outer diameter of 80 to 200 mm defining an outerperiphery, an inner diameter of 30 to 80 mm and a thickness of 0.1 to1.0 mm, and a blade section on the outer periphery of the base, saidblade section comprising diamond and/or CBN abrasive grains pre-coatedwith a magnetic material, a metal or alloy bond formed by electroplatingor electroless plating for bonding abrasive grains together and to thebase, and a resin infiltrated between abrasive grains and betweenabrasive grains and the base, said resin being a thermoplastic resinhaving a melting point of up to 350° C. or a thermoset resin obtained bycuring a liquid thermosetting resin composition having a curingtemperature of up to 350° C.
 2. The cutting wheel of claim 1 wherein theresin is at least one member selected from the group consisting ofacrylic resins, epoxy resins, phenolic resins, polyamide resins,polyimide resins, and modified resins of the foregoing.
 3. The cuttingwheel of claim 1 wherein the resin has a Poisson's ratio between 0.3 and0.48.
 4. The cutting wheel of claim 1 wherein said base has a saturationmagnetization of at least 40 kA/m (0.05 T).
 5. The cutting wheel ofclaim 1 wherein said abrasive grains have an average grain size of 10 to300 μm.
 6. The cutting wheel of claim 1 wherein said abrasive grainshave a mass magnetic susceptibility χg of at least 0.2.
 7. A method formanufacturing an outer blade cutting wheel comprising the steps of:providing a base in the form of an annular thin disc of cemented carbidehaving a Young's modulus of 450 to 700 GPa, having an outer diameter of80 to 200 mm defining an outer periphery, an inner diameter of 30 to 80mm, and a thickness of 0.1 to 1.0 mm, providing diamond and/or CBNabrasive grains pre-coated with a magnetic material, placing a permanentmagnet near the outer periphery of the base so that the magnetic fieldproduced by the permanent magnet may act to magnetically attract andhold the coated abrasive grains close to the outer periphery of thebase, electroplating or electroless plating a metal or alloy on the baseouter periphery and the coated abrasive grains being magneticallyattracted and held, for bonding the abrasive grains together and to thebase to fixedly secure the abrasive grains to the base outer peripheryto form a blade section, and letting a thermoplastic resin having amelting point of up to 350° C. infiltrate into any voids betweenabrasive grains and between abrasive grains and the base or letting aliquid thermosetting resin composition having a curing temperature of upto 350° C. infiltrate into any voids between abrasive grains and betweenabrasive grains and the base and cure thereat.
 8. The method of claim 7wherein the resin is at least one member selected from the groupconsisting of acrylic resins, epoxy resins, phenolic resins, polyamideresins, polyimide resins, and modified resins of the foregoing.
 9. Themethod of claim 7 wherein the infiltrating resin has a Poisson's ratiobetween 0.3 and 0.48.
 10. The method of claim 7 wherein said base has asaturation magnetization of at least 40 kA/m (0.05 T).
 11. The method ofclaim 7 wherein said abrasive grains have an average grain size of 10 to300 μm.
 12. The method of claim 7 wherein said abrasive grains have amass magnetic susceptibility χg of at least 0.2.
 13. The method of claim7 wherein the permanent magnet produces a magnetic field of at least 8kA/m within a space extending a distance of 10 mm or less from the baseouter periphery.